Bacterial neurotoxins (botulinum and tetanus) produced by genus Clostridium are among the most toxic natural substances known. They share many structural and functional similarities, however their sites of action within the nervous system are quite different. Botulinum toxin (BoT) is acting in the periphery, whereas tetanus toxin (TeT) acts in the central nervous system (CNS). It is believed that this phenomenon is due to high affinity binding to different specific protein receptors present at presynaptic membranes. Likely, TeT binds to a receptor which under normal conditions enters the endocytotic pathway and travels along axons while BoT does not. Despite 30 years of research on Clostridial neurotoxins this aspect of their biology remains largely unknown. No protein receptor or non-protein binding molecule has been identified to date which could explain extensive neuronal trafficking of TeT but lack of this feature for homologous BoT molecule(s). The proposed project focuses on a molecular and biochemical analysis of axonal trafficking of TeT. On the base of preliminary results and indirect data existing in the literature, it is proposed that the common neurotrophin p75 receptor may be used as a binding protein and/or axonal carrier for TeT molecules. This project includes a set of experiments to test this hypothesis. Possible interactions between TeT and p75 receptor will be examined in vitro and in vivo under different conditions and in different neuronal cells. Finally, the role of the p75 receptor in TeT axonal trafficking will be functionally probed using neuronal lines in culture with down-regulation of p75 expression as well as in p75 knock-out mouse models. These experiments will clarify the potential role of p75 in TeT neuronal transport. It could be a major advancement in our understanding of neuronal intoxication and may participate in extending knowledge about poorly understood endogenous retrograde axonal pathways used by neurons for delivery of different trophic molecules during development. Highly efficient TeT trafficking pathway through binding to p75 receptor will open an entry gate leading from the periphery to the central nervous system. This may assist in the development of novel modes of delivery of diverse biological agents, including enzymes, analgesics, anesthetic drugs or trophic factors, to the spinal cord and brain stem (Dobrenis et al., 1992; Schneider et al., 2000; Schiavo et al., 2000; Bordet et al., 2001; Kissa et al., 2002).